Method of manufacturing solid electrolytic capacitor

ABSTRACT

A method of manufacturing a solid electrolytic capacitor having an even conductive polymer layer includes the steps of forming a conductive polymer layer on an anode element by bringing a dispersion containing a conductive solid and a first solvent into contact with the anode element having a dielectric film formed thereon, washing the anode element with a second solvent higher in boiling point than the first solvent, in which the conductive solid can be dispersed, after the conductive polymer layer is formed, and drying the anode element washed with the second solvent at a temperature not lower than the boiling point of the first solvent and lower than the boiling point of the second solvent.

This application is a divisional of U.S. patent application Ser. No.13/838,566 filed Mar. 15, 2013, which is a divisional application ofU.S. patent application Ser. No. 12/949,226 filed Nov. 18, 2010, nowU.S. Pat. No. 8,419,809, which is based upon and claims the benefit ofpriority from Japanese Patent Application No. 2009-265322 filed with theJapan Patent Office on Nov. 20, 2009, the entire contents of which beinghereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a solidelectrolytic capacitor, and particularly to a method of manufacturing asolid electrolytic capacitor having a conductive polymer layer, with theuse of a dispersion.

2. Description of the Related Art

As electronic devices have been digitized and adapted to higherfrequencies in recent years, a capacitor having a small size, a largecapacity, and low impedance even in a high-frequency domain has beenrequired. A solid electrolytic capacitor having a conductive polymerlayer has been developed as a capacitor meeting this requirement.Examples of solid electrolytic capacitors include a wound-type solidelectrolytic capacitor and a stack-type solid electrolytic capacitor.For example, Japanese Patent Laying-Open No. 2007-294495 discloses astack-type solid electrolytic capacitor including a metal plate.

For the conductive polymer layer, highly conductive polypyrrole,polythiophene, polyfuran, polyaniline, and the like are employed. Such aconductive polymer layer can be formed, for example, by impregnating ananode element with a polymerization liquid containing a precursormonomer of a conductive polymer, an oxidizing agent, and a dopant andcausing chemical polymerization reaction.

In the conductive polymer layer formed through chemical polymerizationreaction as described above, excessive oxidizing agent or unreactedprecursor monomer may remain. In this case, characteristics of the solidelectrolytic capacitor may disadvantageously be lowered due to abehavior of the remaining oxidizing agent or precursor monomer. It isnoted that a higher capacitance, a lower equivalent series resistance(ESR) value, or a lower leakage current (LC) value can indicate highcharacteristics of the solid electrolytic capacitor.

In addressing the problem above, for example, Japanese PatentLaying-Open No. 2000-106329 discloses a method of washing the conductivepolymer layer with a wash such as ethanol in order to remove theexcessive oxidizing agent remaining in the conductive polymer layer ofthe solid electrolytic capacitor. In addition, Japanese PatentLaying-Open No. 2008-251629 discloses a method of washing the conductivepolymer layer with a wash solution containing a solute in order toremove the unreacted precursor monomer remaining in the conductivepolymer layer of the solid electrolytic capacitor.

In the method of forming the conductive polymer layer by causingchemical polymerization reaction on the anode element as describedabove, polymerization reaction gradually proceeds in a polymerizationliquid from the time point of preparation of the polymerization liquid.Therefore, much attention had to be paid to handling of thepolymerization liquid. In addition, since chemical polymerizationreaction occurs on the anode element, a dielectric film on a surface ofthe anode element may be damaged.

In order to address this, a method of forming a conductive polymer layerwith a dispersion has recently been developed. A dispersion is asubstance obtained by dispersing a conductive solid composed of apolymer in a state of particle or aggregate in a solvent, and forexample, a conductive polymer layer can be formed on an anode element byimmersing the anode element in the dispersion so as to bring theconductive solid and the anode element in physical contact with eachother. For example, Japanese National Patent Publication No. 2004-532298discloses a dispersion in which polythiophene is dispersed. According tothis method, no chemical polymerization reaction occurs on thedielectric film, and hence the dielectric film is not damaged. Inaddition, since no polymerization reaction in the dispersion proceeds asin the case of the polymerization liquid described above, handlingthereof is easy.

It is difficult, however, to achieve a uniform size of particles oraggregates of a conductive polymer contained in the dispersion, andhence large particles or aggregates are present in the dispersion inmany cases. In this case, since a large particle cannot enter the anodeelement, which is a porous body, and evenness of the conductive polymerlayer is lowered. Consequently, characteristics of the solidelectrolytic capacitor are disadvantageously lowered.

SUMMARY OF THE INVENTION

From the foregoing, an object of the present invention is to provide amethod of manufacturing a solid electrolytic capacitor having an evenconductive polymer layer.

The present invention is directed to a method of manufacturing a solidelectrolytic capacitor including an anode element having a dielectricfilm formed on its surface and a conductive polymer layer formed on thedielectric film, including the steps of: forming the conductive polymerlayer on the anode element by bringing a dispersion containing aconductive solid and a first solvent into contact with the anode elementhaving the dielectric film formed thereon; washing the anode elementwith a second solvent higher in boiling point than the first solvent, inwhich the conductive solid can be dispersed, after the conductivepolymer layer is formed; and drying the anode element washed with thesecond solvent at a temperature not lower than the boiling point of thefirst solvent and lower than the boiling point of the second solvent.

Miscibility of a B solvent with an A solvent herein refers to acharacteristic allowing homogeneous mixture of the A solvent and the Bsolvent without application of external force.

According to the present invention, a solid electrolytic capacitorhaving an even conductive polymer layer and a method of manufacturingthe same can be provided.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing a structure of astack-type solid electrolytic capacitor according to one embodiment ofthe present invention.

FIG. 2 is a cross-sectional view schematically showing a structure of acapacitor element in FIG. 1.

FIGS. 3A to 3C are conceptual diagrams for illustrating an effect of awashing step.

FIG. 4 is a flowchart showing a treatment procedure in fabricating aconductive polymer layer in Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described in detail withreference to the drawings. In the embodiment shown below, the same orcorresponding elements have the same reference characters allotted anddescription thereof will not be repeated.

<Solid Electrolytic Capacitor>

Initially, a structure of a solid electrolytic capacitor manufacturedwith a method of manufacturing a solid electrolytic capacitor accordingto the present embodiment will be described. Though the manufacturingmethod is applicable to a chip-type solid electrolytic capacitor havingan anode element made of a sintered object, a wound-type solidelectrolytic capacitor having an anode element made of a metal foil, anda stack-type solid electrolytic capacitor having an anode element madeof a metal plate, description will be given here with reference to astack-type solid electrolytic capacitor.

FIG. 1 is a cross-sectional view schematically showing a structure of astack-type solid electrolytic capacitor according to one embodiment ofthe present invention.

In FIG. 1, a solid electrolytic capacitor 100 includes three capacitorelements 10 a, 10 b and 10 c, and capacitor elements 10 a to 10 cinclude a cathode portion 11 a and an anode portion 12 a, a cathodeportion 11 b and an anode portion 12 b, and a cathode portion 11 c andan anode portion 12 c, respectively. Capacitor elements 10 a to 10 c arestacked such that the cathode portion sides and the anode portion sidesthereof are aligned in the same position.

A cathode terminal 14 is arranged between cathode portion 11 a andcathode portion 11 b, and cathode portion 11 a and cathode terminal 14as well as cathode portion 11 b and cathode terminal 14 are bonded andfixed to each other by an adhesive conductive paste 13. Cathode portion11 b and cathode portion 11 c are also bonded and fixed to each other byconductive paste 13. In addition, an anode terminal 15 is arrangedbetween anode portion 12 a and anode portion 12 b, and anode portion 12a, anode terminal 15, anode portion 12 b, and anode portion 12 c arestacked in this order and press-fitted and fixed to one another.

Then, each portion above is covered with an exterior resin 16 so thatone ends of cathode terminal 14 and anode terminal 15 are exposed.Cathode terminal 14 and anode terminal 15 exposed through exterior resin16 are bent along a side surface and a bottom surface of solidelectrolytic capacitor 100. Though solid electrolytic capacitor 100having three capacitor elements is described in the present embodiment,the number of capacitor elements is not limited to three and it may beset to one or more. In addition, the cathode portion and the anodeportion should only electrically be connected to the cathode terminaland the anode terminal, respectively, and arrangement is not limited tothat in FIG. 1.

FIG. 2 is a cross-sectional view schematically showing a structure ofthe capacitor element in FIG. 1.

In FIG. 2, capacitor element 10 a has an anode element 21 made of avalve metal foil of aluminum, niobium, tantalum, or the like, and adielectric film 22 is formed on a surface of anode element 21.Dielectric film 22 can be formed with a known technique. For example,dielectric film 22 can be formed by immersing a valve metal foil in anaqueous solution of phosphoric acid or the like, applying a prescribedvoltage, and performing chemical conversion treatment. In addition, aconductive polymer layer 23, a carbon layer 24, and a silver paste layer25 are successively stacked on dielectric film 22. Conductive polymerlayer 23, carbon layer 24, and silver paste layer 25 constitute acathode layer. In capacitor element 10 a, a portion where the cathodelayer is stacked on anode element 21 serves as cathode portion 11 a, anda portion where the cathode layer is not stacked but anode element 21 ordielectric film 22 is exposed serves as anode portion 12 a. It is notedthat capacitor elements 10 b and 10 c have the structure the same ascapacitor element 10 a.

<Method of Manufacturing Solid Electrolytic Capacitor>

A method of manufacturing solid electrolytic capacitor 100 will bedescribed hereinafter.

<<Step of Forming Conductive Polymer Layer>>

In the present embodiment, initially, as the step of forming aconductive polymer layer, conductive polymer layer 23 is formed on anodeelement 21 by bringing a dispersion containing a conductive solid and afirst solvent in contact with anode element 21 having dielectric film 22formed thereon.

Here, the conductive solid refers to a substance in which a conductivepolymer having a molecular weight in a range approximately from 1,000 to1,000,000 forms particles or aggregates. Any conductive polymerapplicable to a solid electrolytic capacitor may be adopted as aconductive polymer, and examples thereof include polypyrrole,polythiophene, polyfuran, or polyaniline, or a derivative thereof. Sincepolythiophene or a derivative thereof has high conductivity, a polymercomposed of polythiophene or a derivative thereof is preferred, and inparticular, a conductive polymer composed of polyethylenedioxythiopheneis preferred.

In addition, any solvent in which a conductive solid can be dispersedmay be adopted as a first solvent, and examples thereof include water ora solvent mixture mainly composed of water. Considering ease inhandling, dispersibility of a conductive solid and the like, water ispreferably adopted as a solvent for a dispersion.

Though a method of preparing the dispersion above is not particularlyrestricted, for example, a method of preparing the dispersion bydispersing a conductive solid such as a conductive polymer and the likein the first solvent, a method of obtaining a dispersion containing aconductive solid by polymerizing a monomer which is a precursor of aconductive polymer in the first solvent and synthesizing the conductivepolymer serving as the conductive solid, and the like are available. Inthe case of the latter method, a purification step of removing unreactedmonomer, impurities and the like is preferably provided afterpolymerization reaction.

A method of bringing the dispersion above in contact with anode element21 having dielectric film 22 formed thereon is not particularly limited,and a known method can be used. Among others, a method of immersinganode element 21 in a dispersion accommodated in a container ispreferably used, because an operation is relatively easy.

By immersing anode element 21 in the dispersion and thereafter bringingup anode element 21 from the dispersion, a conductive solid adheres ontoanode element 21, that is, onto dielectric film 22 formed on the surfaceof anode element 21. Not only the conductive solid but also the firstsolvent are present on the surface of anode element 21 immediately afterit is brought up from the dispersion. It is considered that, as thefirst solvent evaporates, the conductive solids are entangled to adhereand consequently conductive polymer layer 23 is formed. Therefore, inorder to rapidly evaporate the first solvent adhering to anode element21 so as to form dense conductive polymer layer 23, anode element 21 ispreferably heated and dried after anode element 21 is brought up fromthe dispersion. For example, in an example where water is employed asthe first solvent, the first solvent can rapidly evaporate by dryinganode element 21 at a temperature around 100° C.

This step of forming a conductive polymer layer can be repeated aplurality of times. By repeating the step a plurality of times, theconductive solid can adhere onto dielectric film 22 densely or to alarge thickness. Consequently, conductive polymer layer 23 achievinghigh characteristics can be formed.

<<Washing Step>>

Then, in the washing step, after conductive polymer layer 23 is formedon anode element 21, anode element 21 is washed with a second solventhigher in boiling point than the first solvent, in which the conductivesolid can be dispersed.

Examples of the second solvent in which the conductive solid can bedispersed include an organic solvent having miscibility with the firstsolvent. If the second solvent has miscibility with the first solvent,the conductive solid can be dispersed in the second solvent, as in thecase of the first solvent. In addition, examples of the second solventin which the conductive solid can be dispersed include a solvent havingviscosity not higher than 100 mPa·s. If the second solvent has viscositynot higher than 100 mPa·s, the conductive solid can readily be dispersedin the second solvent. An organic solvent having viscosity not higherthan 60 mPa·s is more preferred as the second solvent, and an organicsolvent having viscosity not higher than 25 mPa·s is further preferred.In particular, an organic solvent having viscosity not higher than 20mPa·s is preferred.

One of organic solvents having the characteristics above or a mixture oftwo or more of them may be adopted as the second solvent. For example,if water is adopted as the first solvent for the dispersion, forexample, such solvents as γ-butyrolactone, γ-valerolactone, ethylenecarbonate, propylene carbonate, 3-methyl-2-oxazolidone,N-methyl-2-pyrrolidone, N-methyl-2-propylene, 2-ethoxyethanol, ethyleneoxide, and propylene oxide having viscosity not higher than 100 mPa·sand higher in boiling point than water, in which the conductive solidcan be dispersed, can be adopted as the second solvent.

In addition, water may be mixed in the second solvent. If the secondsolvent is a solvent mixture with water, it is considered thatperformance in dispersing the conductive solid is further enhanced. If aratio of mixture of water in the second solvent is too high, however,flowability of the conductive solid which will be described later islowered and hence the ratio of mixture of water is preferably 50% orlower.

A method of washing anode element 21 with the second solvent is notparticularly limited. For example, a method of immersing anode element21 in the second solvent accommodated in a container is preferably used,because an operation is relatively easy.

The present inventor has found that characteristics of solidelectrolytic capacitor 100 can be improved by washing anode element 21having conductive polymer layer 23 formed thereon with the secondsolvent as described above. The reason therefor is unclear, however, ahypothesis as follows can be made.

FIGS. 3A to 3C are conceptual diagrams for illustrating an effect of thewashing step.

As shown in FIG. 3A, generally, a hole 30 is present in the surface ofanode element 21 having dielectric film 22 formed thereon. Hole 30 hererefers to a pit formed as a result of etching of anode element 21 inorder to increase a capacitance of solid electrolytic capacitor 100, anddielectric film 22 is formed also on the surface in hole 30.

Meanwhile, a size of a particle or a size of an aggregate of theconductive solid in a dispersion 31 is varied. In dispersion 31, aconductive solid 32 a in a state of a relatively small particle and aconductive solid 32 b in a state of a particle larger than conductivesolid 32 a are present in a first solvent 33 in a mixed manner.

Therefore, after the step of forming a conductive polymer layerdescribed above, not only conductive solid 32 a but also conductivesolid 32 b adhere to the surface of anode element 21 as shown in FIG.3B. Since conductive solid 32 b is a particle or an aggregate largerthan other conductive solids 32 a, hole 30 in anode element 21 isunevenly filled with large conductive solid 32 b and conductive polymerlayer 23 becomes uneven. In this case, the cathode layer cannot enterhole 30 and come in contact therewith, and a capacitance in a region ofhole 30 cannot be extracted. Therefore, characteristics of solidelectrolytic capacitor 100 are lowered.

In contrast, in the present embodiment, anode element 21 havingconductive polymer layer 23 as shown in FIG. 3B is washed with a secondsolvent 34 described above. Change in conductive polymer layer 23 atthis time is shown in FIG. 3C. As described above, since second solvent34 has a property that conductive solids 32 a and 32 b can be dispersedtherein, large conductive solid 32 b not adhering to anode element 21 isdispersed in second solvent 34 and removed from conductive polymer layer23. Alternatively, large conductive solid 32 b itself can be dispersedin second solvent 34, so that conductive solid 32 b is dispersed againin second solvent 34 and it is reconstructed as a small particle oraggregate. Therefore, it is considered that the conductive solid thathas formed large conductive solid 32 b in FIG. 3C is removed or adheresagain onto anode element 21 as a small particle or aggregate.

In addition, it is possible that conductive solid 32 a within hole 30 islow in a degree of adhesion than conductive solid 32 a arranged in otherportions, due to the presence of large conductive solid 32 b. It isconsidered that second solvent 34 is also able to rearrange conductivesolid 32 a within hole 30 low in a degree of adhesion, and consequently,conductive solid 32 a within hole 30 can be moved and rearranged evenly,to thereby adhere again onto anode element 21.

Such functions as dispersion again/adhesion again or removal of largeconductive solid 32 b and rearrangement/adhesion again of conductivesolid 32 a within hole 30 are considered to be achieved because thesecond solvent is higher in boiling point than the first solvent; inother words, the second solvent is less likely to evaporate than thefirst solvent, and hence flowability of the conductive solid can bemaintained for a long period of time. Such a function can achieve evenconductive polymer layer 23 and resultant improvement in thecharacteristics of the solid electrolytic capacitor.

The present washing step is performed after the step of forming aconductive polymer layer described above. If the step of forming aconductive polymer layer is repeated a plurality of times, however, thewashing step may be interposed at any time during repetition and thewashing step should only be interposed at least once during repetition.In addition, if the washing step is interposed at least once in betweenthe repeated steps of forming a conductive polymer layer, the washingstep is preferably performed in an early stage, that is, in a stagesmall in the number of times of repetition. For example, if the washingstep is performed after the step of forming a conductive polymer layeris performed once and thereafter the step of forming a conductivepolymer layer is repeated, the characteristics of the solid electrolyticcapacitor can effectively be improved.

Further, in the step of forming a conductive polymer layer before thewashing step, it is not necessary to dry the first solvent. In thiscase, however, since a degree of adhesion of the conductive solid to theanode element seems to be low, such adjustment as making shorter a timeperiod for immersing the anode element to which the dispersion adheresin the second solvent is preferably made.

In addition, the present inventor has found that, in manufacturing asolid electrolytic capacitor having a prescribed capacity, the number oftimes of repetition of the step of forming a conductive polymer layer inthe case of performing the present washing step may be smaller than thenumber of times of repetition thereof in the case not performing thepresent washing step. Though the reason therefor is unclear, it isconsidered that a large conductive solid is removed by washing andintroduction of the conductive solid into the hole in the anode elementis facilitated, and consequently, the small number of times ofrepetition of the step of forming a conductive polymer layer, that is,the small number of times of immersion in the dispersion, can stillallow fabrication of a solid electrolytic capacitor having a desiredcapacity.

<<Drying Step>>

Then, in the drying step, anode element 21 is dried at a temperature notlower than the boiling point of the first solvent and lower than theboiling point of the second solvent, after the washing step above.

In this step, only the first solvent that remained on anode element 21is removed and the second solvent present on anode element 21 partiallyremains without being completely removed. Since the remaining secondsolvent is higher in boiling point than the first solvent, the secondsolvent is slowly and gradually removed, for example, in the repeatedstep of forming a conductive polymer layer after the drying step.

Therefore, in the present drying step, conductive polymer layer 23 canbe dried and formed while flowability of the conductive solid ismaintained with the second solvent remaining on anode element 21. It isthus considered that conductive polymer layer 23 can be denser while anuneven portion of conductive polymer layer 23 is reconstructed.

According to the method of manufacturing a solid electrolytic capacitorin the present embodiment described above, evenness of the conductivepolymer layer formed with the dispersion can be improved andconsequently a solid electrolytic capacitor having high characteristicscan be manufactured.

The method of manufacturing a solid electrolytic capacitor according tothe present invention is not limited to manufacturing of a solidelectrolytic capacitor according to the embodiment above, and it isapplicable to a method of manufacturing a solid electrolytic capacitorin known other forms. Examples of known other forms include a chip-typesolid electrolytic capacitor, a wound-type solid electrolytic capacitor,and the like. In the case of a chip-type solid electrolytic capacitor, adispersion is brought in contact with a sintered object having adielectric film thereon and thereafter the sintered object can be washedwith a second solvent. In the case of a wound-type solid electrolyticcapacitor, after a wound body formed by winding an anode foil and acathode foil with a separator being interposed therebetween isimpregnated with a dispersion, the wound body can be washed with asecond solvent.

EXAMPLES Example 1

<<Preliminary Preparation>>

(1) Preparation of Dispersion

A solution mixture obtained by dissolving 3,4-ethylenedioxythiophene andpolystyrene sulfonate serving as a dopant in ion exchanged water servingas the first solvent was prepared. While stirring the obtained solutionmixture, a ferric p-toluenesulfonate solution serving as the oxidizingagent dissolved in ion exchanged water was added thereto, to therebyobtain a reaction liquid. Then, the obtained reaction liquid wasdialyzed to remove unreacted monomer and excessive oxidizing agent, tothereby obtain a solution containing polyethylenedioxythiophene dopedwith approximately 5 mass % polystyrene sulfonate. Then, imidazole washomogeneously dispersed in this solution, to thereby obtain thedispersion.

(2) Fabrication of Anode Element Having Dielectric Film Thereon

Chemical conversion treatment was performed by immersing a large-sizedaluminum foil having a thickness of 0.1 mm in an ammonium adipateaqueous solution and applying a voltage of 3 V, to thereby form analuminum oxide film serving as the dielectric film on the surface of thealuminum foil. Then, this aluminum foil was cut into a size of 6 mmlong×3.5 mm wide, and the cut surface was subjected to chemicalconversion treatment as described above, to thereby fabricate the anodeelement.

<<Fabrication of Conductive Polymer Layer>>

Then, the fabricated anode element and the prepared dispersion were usedto fabricate a conductive polymer layer. FIG. 4 is a flowchart showing atreatment procedure in fabricating a conductive polymer layer inExample 1. Fabrication of the conductive polymer layer will be describedhereinafter with reference to FIG. 4.

(1) Step of Forming Conductive Polymer Layer (First Time)

Initially, in step S40 in FIG. 4, the anode element was immersed in thedispersion for 5 minutes and thereafter the anode element was brought upfrom the dispersion at a rate of 1 mm/s. Then, the brought-up anodeelement was placed in an oven and dried for 10 minutes at 100° C.

(2) Washing Step

Then, in step S41, the dried anode element was immersed in propylenecarbonate (PC), which is the second solvent, and thereafter the anodeelement was brought up from propylene carbonate at a rate of 1 mm/s.

(3) Drying Step

Then, in step S42, the anode element brought up from propylene carbonatewas placed in an oven and dried for 10 minutes at 150° C.

(4) Step of Forming Conductive Polymer Layer (Second to Fourth Times)

Then, in step S43, the treatment the same as in the step of forming aconductive polymer layer (first time) was repeated three times.Therefore, finally, an operation for immersing the anode element in thedispersion was performed four times.

<<Fabrication of Other Components>>

A carbon layer and a silver paste layer were stacked with a known methodon the conductive polymer layer fabricated in the steps above, tothereby fabricate a capacitor element. Three such capacitor elementswere fabricated. As shown in FIG. 1, the capacitor elements werestacked, the cathode terminal was arranged between the cathode portions,and adjacent members were bonded and fixed to each other by interposinga conductive paste between the cathode portion and the cathode terminalof the three stacked capacitor elements. In addition, the anode terminalwas arranged between the anode portions of the capacitor elements andthe anode portion and the anode terminal of the three stacked capacitorelements were press-fitted and formed.

Then, epoxy resin was used as the exterior resin, and each member wascovered by using a transfer molding method so as to expose one ends ofthe cathode terminal and the anode terminal. Then, the cathode terminaland the anode terminal exposed through the exterior resin were bentalong the side surface and the bottom surface of the molded exteriorresin and dicing was finally performed. The solid electrolytic capacitorstructured as shown in FIG. 1 was thus completed.

Example 2

The solid electrolytic capacitor was fabricated in accordance with thetreatment procedure as in Example 1, except for using propylenecarbonate containing 50 wt % water as the second solvent.

Example 3

The solid electrolytic capacitor was fabricated in accordance with thetreatment procedure as in Example 1, except that the washing step wasperformed not only after the step of forming a conductive polymer layerfor the first time but also after the step of forming a conductivepolymer layer for the second time.

Examples 4 to 7

The solid electrolytic capacitors were fabricated in accordance with theprocedure as in Example 1, except for using as the second solvent,γ-butyrolactone (γ-BL) in Example 4, N-methyl-2-pyrrolidone (NMP) inExample 5, ethylene glycol (EG) in Example 6, and propylene glycol (PG)in Example 7, respectively.

Example 8

The solid electrolytic capacitor was fabricated in accordance with theprocedure as in Example 1, except that ethylene glycol monoethyl ether(EGmEE) was employed as the second solvent and the drying step after thewashing step was performed at 100° C.

Comparative Example 1

The solid electrolytic capacitor was fabricated in accordance with theprocedure as in Example 1, except that the step of forming a conductivepolymer layer was repeated seven times without performing the washingstep.

Comparative Example 2

The solid electrolytic capacitor was fabricated in accordance with theprocedure as in Example 1, except for using ethanol as the secondsolvent.

Comparative Example 3

The solid electrolytic capacitor was fabricated in accordance with theprocedure as in Comparative Example 2, except that the step of forming aconductive polymer layer was repeated six times after the drying step.Therefore, finally, an operation for immersing the anode element in thedispersion was performed seven times.

Comparative Example 4

The solid electrolytic capacitor was fabricated in accordance with theprocedure as in Comparative Example 3, except that the drying step afterthe washing step was performed at 100° C. Therefore, finally, anoperation for immersing the anode element in the dispersion wasperformed seven times.

Comparative Example 5

The solid electrolytic capacitor was fabricated in accordance with theprocedure as in Example 8, except that the drying step after the washingstep was performed at 150° C. and the step of forming a conductivepolymer layer after the drying step was repeated six times. Therefore,finally, an operation for immersing the anode element in the dispersionwas performed seven times.

Comparative Example 6

The solid electrolytic capacitor was fabricated in accordance with theprocedure as in Example 1, except that 1,5-pentanediol (1,5-PD) havinghigh viscosity was employed as the second solvent and the step offorming a conductive polymer layer after the drying step was repeatedsix times. Therefore, finally, an operation for immersing the anodeelement in the dispersion was performed seven times.

<Performance Evaluation>

<<Capacitance>>

Capacitance (μF) at a frequency of 120 Hz, of 50 solid electrolyticcapacitors in each Example and each Comparative Example was measured byusing an LCR meter for 4-terminal measurement. An average value ofmeasurement results was calculated.

<<ESR Value>>

ESR (mΩ) at a frequency of 100 kHz, of 50 solid electrolytic capacitorsin each Example and each Comparative Example was measured by using anLCR meter for 4-terminal measurement. An average value of measurementresults was calculated.

<<LC Value>>

A rated voltage was applied for 2 minutes to 50 solid electrolyticcapacitors in each Example and each Comparative Example and a leakagecurrent amount of each solid electrolytic capacitor thereafter wasmeasured. It is noted that the rated voltage of each solid electrolyticcapacitor was 2.0 V.

For facilitating comparison among Examples 1 to 8 and ComparativeExamples 1 to 6 above, Table 1 summarizes types of the second solventsused in the Examples, properties of the second solvents, manufacturingconditions, and results of measured values, and Table 2 summarizes typesof the second solvents used in the Comparative Examples, properties ofthe second solvents, manufacturing conditions, and results of measuredvalues.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Example 8 Organic Type PC PC + PC γ-BL NMP EG PG EGmEE SolventWater Viscosity 1.4 — 1.4 1.7 1.7 25.7 56 2.1 (mPa · s) Boiling Point242 — 242 204 202 197 187.4 136 (° C.) Manufacturing The Number 1 1 2 11 1 1 1 Condition of Times of Washing (times) Drying 150 150 150 150 150150 150 100 Temperature (° C.) Total Number 4 4 4 4 4 4 4 4 of Times ofImmersion (times) Performance Capacitance 96.9 101.4 103.2 97.8 95.498.7 94.5 98.4 (μF) ESR Value 13.9 12.2 13 13.4 13.5 14.3 14.5 13.7 (mΩ)LC Value 2.9 2.7 1.5 2.1 2.2 3.3 3.1 2.9 (μA)

TABLE 2 Comparative Comparative Comparative Comparative ComparativeComparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Organic Type — Ethanol Ethanol Ethanol EGmEE 1.5-PD Solvent Viscosity(mPa · s) — 1.1 1.1 1.1 2.1 128 Boiling Point (° C.) — 78.3 78.3 78.3136 238 Manufacturing The Number of — 1 1 1 1 1 Condition Times ofWashing (times) Drying Temperature — 150 150 100 150 150 (° C.) TotalNumber of 7 4 7 7 7 7 Times of Immersion (times) Performance Capacitance(μF) 88.2 59.1 60.9 89.1 70.5 77.7 ESR Value (mΩ) 16.3 23.6 20.4 19.619.9 24.5 LC Value (μA) 7.2 43.2 15.8 9.2 10.8 36

When Example 1 and Comparative Example 1 are compared with each other,the result exhibited that the capacitance was higher, the ESR value waslower, and the LC value was lower, that is, the characteristics of thesolid electrolytic capacitor were higher by performing the washing stepusing propylene carbonate than in the case where the washing step wasnot performed, despite the smaller number of times of immersion in thedispersion. This may be because, evenness of the conductive polymerlayer was improved by washing the conductive polymer layer withpropylene carbonate in which the conductive solid can readily bedispersed and by drying the washed anode element at a temperature higherthan the boiling point of water which is the first solvent and lowerthan the boiling point of propylene carbonate. Therefore, by performingthe washing step, not only characteristics of the solid electrolyticcapacitor are improved but also the manufacturing process can besimplified.

In addition, when Example 1 and Example 2 are compared with each other,the result exhibited that characteristics of the solid electrolyticcapacitor were higher in the example where water was contained inpropylene carbonate than in the example where propylene carbonate wasused in the washing step. This may be because propylene carbonatecontaining water has higher dispersibility of the conductive solid andhigher miscibility with water on the anode element.

Moreover, when Example 1 and Example 3 are compared with each other, theresult exhibited that characteristics of the solid electrolyticcapacitor were higher in the example where the washing step wasperformed twice than in the example where the washing step was performedonce in between the repeated steps of forming a conductive polymerlayer. This may be because efficiency in removal or dispersion again ofa large conductive solid as shown in FIG. 3 was enhanced.

Further, it is considered that characteristics of the solid electrolyticcapacitor were higher in the example where the washing step is performedafter performing the step of forming a conductive polymer layer once asin Example 1 than in the example where the washing step is performedafter performing the step of forming a conductive polymer layer aplurality of times. This is because densification or the like of theconductive polymer layer by repeating the step of forming a conductivepolymer layer is efficiently achieved by suppressing in an early stage,filling of the hole in the anode element with a large conductive solid.

Furthermore, it was found from Examples 4 to 8 that characteristics ofthe manufactured solid electrolytic capacitor are improved by performingthe washing step with the use of various second solvents higher inboiling point than water, in which the conductive solid can readily bedispersed.

In addition, when Example 1 is compared with Comparative Example 2, itwas found that characteristics of the solid electrolytic capacitor waspoorer in the example where ethanol was employed as the second solventthan in the example where propylene carbonate was employed. Moreover,the solid electrolytic capacitor in Comparative Example 2 was lower incharacteristics than the solid electrolytic capacitor in ComparativeExample 1. It is considered that a second solvent extremely high inmolecular polarity such as ethanol is poor in substantial dispersibilityof the conductive solid on the anode element. Further, it is consideredthat ethanol cannot maintain flowability of the conductive solid on theanode element for a long period of time because ethanol is lower inboiling point than water.

Furthermore, characteristics of the solid electrolytic capacitor couldnot be improved as in Examples 1 to 8 when ethanol was used as thesecond solvent, even though the number of times of performing the stepof forming a conductive polymer layer is increased as in ComparativeExample 3, or the number of times of performing the step of forming aconductive polymer layer is increased and a drying temperature in thedrying step following the washing step is lowered as in ComparativeExample 4.

When Example 8 is compared with Comparative Example 5, it was found thatcharacteristics of the solid electrolytic capacitor were poorer in theexample where the drying temperature after the washing step was equal toor higher than the boiling point of the second solvent than in theexample where it was lower than the boiling point of the second solvent.In addition, the solid electrolytic capacitor according to ComparativeExample 5 was poorer in characteristics than the solid electrolyticcapacitor according to Comparative Example 1. Therefore, it was foundthat it is important to perform the drying step following the washingstep at a temperature lower than the boiling point of the secondsolvent.

Further, 1,5-pentanediol having viscosity exceeding 100 mPa·s wasemployed in Comparative Example 6, and characteristics of the solidelectrolytic capacitor here were lower than in Example 1 and lower thanin Comparative Example 1. This may be because substantial dispersibilityof the conductive solid on the anode element is poor when the secondsolvent has viscosity exceeding 100 mPa·s. In addition, it is consideredthat it is also difficult to maintain flowability of the conductivesolid if viscosity is equal to higher than 100 mPa·s.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the scopeof the present invention being interpreted by the terms of the appendedclaims.

What is claimed is:
 1. A method of manufacturing an electrolyticcapacitor comprising the steps of: adhering particles made of conductivepolymer or aggregates made of conductive polymer onto an anode elementby bringing a dispersion in which said particles made of conductivepolymer or said aggregates made of conductive polymer are dispersed inthe first solvent into contact with said anode element having adielectric layer formed thereon; bringing a second solvent whichdisperses said particles made of conductive polymer or said aggregatesmade of conductive polymer into contact with said particles made ofconductive polymer adhered onto said anode element or said aggregatesmade of conductive polymer adhered onto said anode element; removing atleast one part of said first solvent, said second solvent, or said firstsolvent and said second solvent after said second solvent is broughtinto contact with said particles made of conductive polymer or saidaggregates made of conductive polymer.
 2. The method of manufacturing anelectrolytic capacitor according to claim 1, wherein in said step ofadhering said particles made of conductive polymer or said aggregatesmade of conductive polymer onto said anode element, at least one part ofsaid first solvent is removed after said dispersion is brought incontact with said anode element having said dielectric layer formedthereon.
 3. The method of manufacturing an electrolytic capacitoraccording to claim 1, wherein said second solvent is brought intocontact with said particles made of conductive polymer or saidaggregates made of conductive polymer without removing said firstsolvent.
 4. The method of manufacturing an electrolytic capacitoraccording to claim 1, wherein said step of adhering said particles madeof said conductive polymer or said aggregates made of said conductivepolymer onto said anode element is repeated a plurality of times, andthe step of bringing said second solvent into contact with saidparticles made of conductive polymer or said aggregates made ofconductive polymer is performed at least once in between the repeatedsteps.
 5. The method of manufacturing an electrolytic capacitoraccording to claim 1, wherein said first solvent is water or a solventmixture mainly composed of water.
 6. The method of manufacturing anelectrolytic capacitor according to claim 1, wherein the boiling pointof said second solvent is higher than the boiling point of the firstsolvent.
 7. The method of manufacturing an electrolytic capacitoraccording to claim 2, wherein the boiling point of said second solventis higher than the boiling point of said first solvent.
 8. The method ofmanufacturing an electrolytic capacitor according to claim 3, whereinthe boiling point of said second solvent is higher than the boilingpoint of said first solvent.
 9. The method of manufacturing anelectrolytic capacitor according to claim 1, wherein said second solventhas miscibility with said first solvent.
 10. The method of manufacturingan electrolytic capacitor according to claim 2, wherein said secondsolvent has miscibility with said first solvent.
 11. The method ofmanufacturing an electrolytic capacitor according to claim 3, whereinsaid second solvent has miscibility with said first solvent.
 12. Themethod of manufacturing an electrolytic capacitor according to claim 1,wherein said second solvent has viscosity not higher than 100 mPa·s. 13.The method of manufacturing an electrolytic capacitor according to claim2, wherein said second solvent has viscosity not higher than 100 mPa·s.14. The method of manufacturing an electrolytic capacitor according toclaim 3, wherein said second solvent has viscosity not higher than 100mPa·s.
 15. The method of manufacturing an electrolytic capacitoraccording to claim 1, wherein said second solvent contains at least oneof γ-butyrolactone, γ-valerolactone, ethylene carbonate, propylenecarbonate, 3-methyl-2-oxazolidone, N-methyl-2-pyrrolidone,N-methyl-2-propylene, 2-ethoxyethanol, ethylene oxide, ethylene glycol,propylene oxide, propylene glycol, and ethylene glycol mono ethyl ether.16. The method of manufacturing an electrolytic capacitor according toclaim 2, wherein said second solvent contains at least one ofγ-butyrolactone, γ-valerolactone, ethylene carbonate, propylenecarbonate, 3-methyl-2-oxazolidone, N-methyl-2-pyrrolidone,N-methyl-2-propylene, 2-ethoxyethanol, ethylene oxide, ethylene glycol,propylene oxide, propylene glycol, and ethylene glycol mono ethyl ether.17. The method of manufacturing an electrolytic capacitor according toclaim 3, wherein said second solvent contains at least one ofγ-butyrolactone, γ-valerolactone, ethylene carbonate, propylenecarbonate, 3-methyl-2-oxazolidone, N-methyl-2-pyrrolidone,N-methyl-2-propylene, 2-ethoxyethanol, ethylene oxide, ethylene glycol,propylene oxide, propylene glycol, and ethylene glycol mono ethyl ether.18. The method of manufacturing an electrolytic capacitor according toclaim 1, wherein said conductive polymer contains at least one ofpolypyrrole, polythiophene, polyfuran, and polyaniline, and a derivativethereof.
 19. The method of manufacturing an electrolytic capacitoraccording to claim 1, wherein said conductive polymer containspolyethylenedioxythiophene and polystyrene sulfonate.